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       __local - Address Space Qualifier.

       __local local


       OpenCL implements the following disjoint address spaces: __global, __local, __constant,
       and __private The address space qualifier may be used in variable declarations to specify
       the region of memory that is used to allocate the object. The C syntax for type qualifiers
       is extended in OpenCL to include an address space name as a valid type qualifier. If the
       type of an object is qualified by an address space name, the object is allocated in the
       specified address name; otherwise, the object is allocated in the generic address space.

       The address space names without the __prefix i.e.  global, local, constant and private may
       be substituted for the corresponding address space names with the __prefix.

       The generic address space name for arguments to a function in a program, or local
       variables of a function is __private. All function arguments shall be in the __private
       address space.

       functionQualifiers(3clc) function arguments declared to be a pointer of a type can point
       to one of the following address spaces only: __global, __local or __constant. A pointer to
       address space A can only be assigned to a pointer to the same address space A. Casting a
       pointer to address space A to a pointer to address space B is illegal.

       Function arguments of type otherDataTypes(3clc), otherDataTypes(3clc),
       otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), and otherDataTypes(3clc)
       refer to image memory objects allocated in the __global address space.


       The __local or local address space name is used to describe variables that need to be
       allocated in local memory and are shared by all work-items of a work-group. Pointers to
       the __local address space are allowed as arguments to functions (including __kernel
       functions). Variables allocated in the __local address space can also be defined inside a
       __kernel function scope.

       There is no generic address space name for program scope variables. All program scope
       variables must be declared in the __constant address space. For example:

       // declares a pointer p in the __private address
       space that // points to an int object in address
       space __global __global int *p;

       // declares an array of 4 floats in the
       __private address space.  float x[4];

       There is no address space for function return values. Using an address space qualifier in
       a function return type declaration will generate a compilation error, unless the return
       type is declared as a pointer type and the qualifier is used on the points-to address

       For example:

       __private int f() { ... } // should generate an
       error __local int *f() { ... } // allowed
       __local int * __private f() { ... }; // should
       generate an error.

       // Examples of variables allocated in the
       __local address space // inside a __kernel

       __kernel void my_func(...)  { __local float
       a;       // A single float allocated // in local
       address space

       __local float      b[10];   // An array of 10
       floats // allocated in local address space.  if
       (...)  { // example of variable in __local
       address space but not // declared at __kernel
       function scope.  __local float c;       // not
       allowed.  } }

       Variables allocated in the __local address space inside a __kernel function cannot be

           __kernel void my_func(...)
           local float a = 1;    // not allowed __local float b; b = 1;                // allowed

       NOTE: Variables allocated in the __local address space inside a __kernel function are
       allocated for each work-group executing the kernel.


       OpenCL Specification[1]


       global(3clc), constant(3clc), private(3clc)


       The Khronos Group


       Copyright © 2007-2011y The Khronos Group Inc.
       Permission is hereby granted, free of charge, to any person obtaining a copy of this
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       without restriction, including without limitation the rights to use, copy, modify, merge,
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       portions of the Materials.


        1. OpenCL Specification
           page 222, section 6.5 - Address Space Qualifiers